Planet Hunters Bag Systems With Super-Earths and Double Saturns

The planet-hunting Kepler spacecraft has discovered two Saturn-like planets orbiting a single star. The new planetary system, the first in which both planets can be seen crossing in front of their star, may also contain an Earth-sized companion.

The Aug. 26 announcement came on the heels of another explosive exoplanet discovery: a system that contains up to seven planets, a new record for exoplanet systems. Both discoveries are being discussed at the international colloquium “Detection and dynamics of transiting exoplanets” at the Observatoire de Haute-Provence, France.

“It’s hard to choose between these two very exciting results,” said exoplanet expert Josh Winn of MIT, who is attending the conference in France but was not involved in either new work.

The possible seven-planet system orbits a sun-like star called HD 10180, located 127 light-years away in the constellation Hydrus. Six years of observations revealed the presence of five planets between 13 and 25 times the mass of Earth, or about the mass of Neptune, that orbit the parent star once every 6 to 600 days. An international team of astronomers led by Christophe Lovis of the Geneva Observatory in Switzerland presented the finding on Aug. 24, and have submitted a paper (.pdf) to the journal Astronomy & Astrophysics.

The data also strongly hint at the presence of two more planets. One would be Saturn-sized, with a minimum mass of 65 Earth masses. The other, at just 1.4 Earth masses, would be the least massive exoplanet yet, if confirmed.

Aside from the sheer number of planets, the cool thing about this system is that it is “full” — “that is, the planets are about as close to one another as they could be, without causing the system to become unstable,” Winn said. “This paper argues that not only the newest system, but also many of the previously detected systems, are ‘full’ and therefore that this arrangement is common.”

The team found and confirmed the new planets using the tried and true radial velocity technique, which watches for the parent star’s tiny movements as interactions with the planets’ gravity tug the star to and fro. This method is the most common technique for finding exoplanets; in fact, the instrument the team used, the High Accuracy Radial Velocity Planet Searcher attached to a 3.6-meter telescope in La Silla, Chile, is the most prolific planet-hunting device out there.

The new Kepler system, on the other hand, was confirmed using a completely new technique that planet hunters have been anxious to try for years.

“The demonstration that this technique works” is important, Winn said. “It has been held out for more than 5 years as a promising technique in theory, but this is the first time it has been put into practice.”

The Kepler Space Telescope stares unblinkingly at a single patch of sky and watches for planets that cross in front of their stars, or transit. The planet’s crossing blocks some of the star’s light, making the star dim periodically.

If the star hosts just one transiting planet, that dimming should happen like clockwork. But the new system, dubbed Kepler-9, is the first system that is known for sure to have two transiting planets. And the pair of planets are never on time. They cross in front of their star about once every 19.24 and 38.91 days, but those times can vary by up to 4 minutes for the inner planet and 39 minutes for the outer one. The planets’ gravitational interactions with each other mean that their transits are sometimes early, sometimes late.

These time differences alone were enough to confirm that these bodies are definitely planets, and not brown dwarfs or other stars.

The transit timing variations also let the Kepler team determine the planets’ masses, a measurement that Kepler can’t usually make. Transit observations can help measure the planet’s radius — the bigger the planet, the dimmer the star — which can help determine the planet’s composition and give clues about how it formed. But to get the planet’s mass, astronomers normally need to follow up with time-consuming radial velocity observations at some of the planet’s biggest telescopes.

“The great part about this method is it comes for free,” said exoplanet astronomer Eric Ford, a co-author of a paper describing Kepler-9 published online Aug. 26 in Science Express. “We’ll be able to identify small planets around faint stars without resorting to observations from world’s largest telescopes. It will be a real boost for the mission.”

Just to be sure, the team followed up with six radial velocity observations at the Keck I telescope in Hawaii. The combined observations showed that both planets are about 0.8 times the radius of Jupiter, or slightly smaller than Saturn, and their masses are about 0.25 and 0.17 times the mass of Jupiter. Theoretical models suggest that both are gas giants with rocky cores.

The Kepler data also give a tantalizing hint of a third transiting planet that sits scorchingly close to the star.

“I think it’s a planet, most of the Kepler team thinks it’s a planet, but it’s not at this stage confirmed,” said astronomer Daniel Fabrycky of the Harvard-Smithsonian Center for Astrophysics, a co-author of the new paper.

At just 1.5 times the radius of the Earth, this planet (if it’s confirmed) would be the smallest yet discovered. It orbits the star once every 1.6 days, meaning its surface temperature is upwards of 3500 degrees Fahrenheit. The planet is probably rocky, Fabrycky said, but any water or atmosphere it may once have had would have evaporated early in its lifetime.

The planet is so close to the star that one side is probably locked facing the star, the same way the moon always shows the same face to the Earth. It’s unlikely that life could survive on such a hot world, but any aliens that made it would live on the night side, Fabrycky said. If they looked up, they would see the two Saturns crossing the sky every 1.6 days, appearing half as large as the moon. But because the Saturns would rise at different times every night, the hypothetical alien civilization would have a hard time using its companion planets to tell time or create a calendar.

A paper analyzing the probability that the new candidate is really a planet is expected on the preprint server arXiv.org tonight, though it will take more observations to know for sure. In the meantime, Kepler will keep watching for other Earth-sized planets that may support life.

“Mankind has been asking the question, ‘Are there other planets out there? Is there other life out there?’ for thousands of years,” said Kepler principal investigator William Borucki in a press teleconference Thursday. “We’re now in a very exciting period, where in the next few years we’ll have the answers to some of these questions.”